Recently, in a vehicle, an air bag device, which is one of a safety device, has become a standard equipment in order to ensure safety of an occupant in case of a traffic accident. An impact generated on the vehicle by a collision is detected by a G-sensor. Based on a signal from the G-sensor, an operating signal for operating the air bag device is inputted to the air bag device which is provided in the center of a steering wheel for a driver or a dashboard panel of a passenger seat. When the operating signal is inputted to the air bag device, an inflator accommodated in the air bag device is ignited. By this ignition, gas for inflating the air bag is expanded. Then the air bag is quickly inflated by the gas, and the occupant sitting on a front seat is protected.
For example, a known air bag device is disclosed in a Japanese Patent Laid-Open Publication No. 2001-239875. The air bag device has four load detecting sensors for detecting the weight of an occupant sitting on a seat. Based on output load values detected by the four load detecting sensors, a load detection value is calculated by a CPU. Then the type of occupant sitting on the seat is determined by comparing the load detection value with each predetermined threshold.
However, in the air bag device disclosed in the foregoing publication, the load detection signal or the output load value detected by the load detecting sensor is easily affected by noise. Thus, the influence of the noise on the load detection signal for detecting the occupant sitting on the seat has to be prevented. To prevent the influence of the noise (for example, variation of the load dection signal), a low-pass filter, which performs filter calculation based on a previous load detection signal and a current load detection signal, and a delay circuit for delaying the load detection signal are generally applied.
Particularly, in a control device having a low consumption mode (sleep mode) for reducing power consumption when the occupant on the seat does not need to be detected, when an operation mode is switched from the low consumption mode to a normal operation mode (normal mode), the load detection signal detected by the load detecting sensor is unsteady and can be easily affected by the noise. Further, in the foregoing structure, when the variation of the load detection signal is prevented by the low-pass filter performing the filter calculation based on two detected signals or more (for example, the previous load detection signal and the current load detection signal), as shown in FIG. 8(a), the load detection signal detected by the load detecting sensor gradually increases until the signal reaches a value corresponding to an actual load applied to the seat, and thus rising time of the low-pass filter delays.
When the foregoing control device is applied to the air bag device, it takes time for the load detection signal to reach the value corresponding to the actual load applied to the seat. Thus, the time elapsing until the air bag device operates is delayed because of the time lag associated with the time elapsing until the occupant sitting on the seat is detected, which is not preferable.
The present invention therefore seeks to provide an occupant weight detecting device capable of stably and quickly detecting actual load applied to a seat after an operation mode is switched to a normal mode.